This invention relates to optical fiber ribbons and, more particularly, to a ribbon bonding matrix arrangement for facilitating access to fibers and fiber subunits in the ribbon.
Optical fiber cable development, wherein the cable is capable of multi-channel transmission, has resulted in the use of bonded arrays of fibers forming a planar ribbon, which may be stacked within a core tube or sheath to produce high operative capacity. Typically, a ribbon may comprise, for example, twelve fibers that are held in a spaced parallel array by a matrix. Splicing and connecting the individual fibers can be accomplished by splicing and connecting the entire planar ribbon in one operation provided that the fibers are positioned and fixed within the matrix.
In U.S. Pat. No. 4,900,126 of Jackson, et al., the disclosure of which is incorporated herein by reference, there is shown a bonded optical fiber ribbon that comprises a coplanar array of longitudinally extending parallel optical fibers in contact with each other. Each fiber is enclosed in inner and outer layers of coating materials and has a color identifier for differentiating each fiber from the other fibers. The inner layer comprises an ultra-violet curable bonding material having a modulus of approximately 1 MPa and an outer layer of an ultra-violet curable bonding material having a modulus of approximately 1 GPa for mechanical protection. With the fibers disposed in a parallel array, interstices are created between the fibers themselves and between the fibers and the envelope of the ribbon, which is a matrix formed of an ultra-violet curable bonding material having a modulus that is less than the modulus of the outer coating layer on the fiber and which is greater than the modulus of the modulus of the inner coating layer. The matrix material fills the interstices and bonds the fibers together and to the envelope to form a completed ribbon. The modulus and the surface characteristics of the matrix material and its bond to the color identifier material on each fiber are such that inter-ribbon movement can occur, thus the ribbons in a stack can slide relative to each other when the cable is bent and, also, that accessing of individual fibers is possible. The ribbons may be stacked such that eighteen ribbons, for example, having twelve fibers each, may be enclosed within a core tube to form the core of an optical fiber cable having two hundred and sixteen fibers, or, if preferred, channels. The core tube itself has an outside diameter (OD) of approximately 0.6 inches. Such an arrangement, which is in widespread use today, and now has extended to the design and manufacture of 864 and larger ribbon cables has proved adequate for most present day applications.
As pointed out in the aforementioned Jackson et al. patent, the color identifier material of each of the fibers should not be removed from the fiber when the bonding material is removed to access the fibers. Thus, the matrix material of the bonded ribbons is selected to have an interfacial bonding characteristic such that the bond interface of the matrix material and the coloring material is weaker than the bonding interface of the coloring material to the outermost coating on the optical fiber. In at least one embodiment of the invention of that patent, a release agent is applied over the coloring material prior to application of the matrix bonding material to facilitate separation of the matrix material therefrom. There remains a problem, however, in breaking out separate modules from the ribbon, and individual fibers from the module. In general, when it is desired to break out one or more modules from the ribbon, and one or more fibers from the module, a matrix cutting tool is used. Such a tool usually comprises a metallic blade having a cutting edge for slicing through the matrix, however, with such a tool, extreme care must be exercised to avoid nicking or otherwise damaging the fiber or fibers adjacent to the cut. Where, as is the case with the ribbon of the aforementioned Jackson et al. patent, the individual fibers are in actual contact with each other, the straight cutting edge of the blade is almost certain to contact the fiber, and avoidance of damage is extremely difficult. This problem is compounded by the fact that most such xe2x80x9cbreakoutsxe2x80x9d are performed in the field, under less than ideal conditions, and the installer or splicer is forced to proceed slowly and with extreme care. In addition, where, in a sixteen fiber ribbon, for example, the ribbon is divided into four modules of four fibers each, it is quite difficult to identify the line of separation between modules, and to cut along that line.
In U.S. Pat. No. 6,134,364 of DeFabritis, et al, the disclosure of which is incorporated herein by reference, there is shown an optical fiber ribbon in which the matrix material has certain characteristics, to be discussed in detail hereinafter, which enhance fiber access by increasing the strippability of the ribbon. With the increased strippability, individual fibers or groups thereof can be broken out while minimizing potential damage to the fibers.
The optical fiber network is moving more and more closely to the end user. Thus, the numbers of fibers required for optimal routing and interfacing with the lower or final stages of the network is considerably smaller than the number used for the main or trunk portion of the network. In the ultimate limit, a single two or four fiber ribbon may be all that is needed for the end user, depending on the level of redundancy used. However, lower fiber count ribbons present problems in production, primarily because economy of manufacture, e.g., high productivity is directly proportional to the ribbon fiber count. This relationship results from the fact that maximum ribbon line speed (in production) is limited by the flow dynamics and cure speed of the matrix, both of which are relatively independent of the fiber count per ribbon. An additional problem can arise in the packaging of the ribbons. A low fiber count ribbon being wound on a ribbon take-up spool can exhibit an instability, at least partially because of its narrowness, which can and often does lead to fiber breakage on de-spooling.
Thus, the DeFabritis arrangement is excellent for breaking out fibers from the ribbon, but does not completely satisfy the needs presented by low fiber count cables.
The currently used modular ribbon design is not suitable for continuous separation into long lengths ( greater than km) of sub-ribbons that are needed for cables because the low adhesions required for single fiber access render such sub-ribbons mechanically unsuitable for cabling after separation. The requisite robustness cannot be achieved by increasing the adhesion of the matrix to the colored fibers without risk that the glass fiber itself will be separated from its protective coating during separation of the sub ribbons.
The present invention is a composite modular ribbon structure that comprises a plurality of optical fibers encased within a multiple-layer matrix. The structure and properties of the matrices are suitably chosen to allow high-speed separation of the composite structure into long lengths of robust sub-ribbons such as are typically required for cabling operations. The composite ribbon is sufficiently wide enough to form a robust, stable package on the take-up spool for cabling and ribbon transport.
In greater detail, in a preferred embodiment of the invention, the outermost matrix which may or may not occupy the intervening space between the sub-ribbons has low thickness, low modulus, low surface friction and narrower, and low tear strength, and has sufficient adhesion to the inner matrix forming the encased sub-ribbons so that their separation is achieved by tearing through the thin web formed by the envelope, thereby leaving a thin layer of the outer matrix bonded to the sub-ribbons. The outer matrix may be envisioned as a tough but easily tearable packaging material capable of being rendered with suitably low surface coefficient of friction such as the various types of plastic wraps for use in domestic kitchens. The primary desideratum of the outer matrix is that it is easily torn for separating out sub-ribbons at the high speeds typical of cabling operations. Such ease of tearing may be obtained with various means by those skilled in the art of material formulation which are not limited to just the material properties listed in the preferred embodiment. Thus, a plurality of low fiber count sub-ribbons, each containing an arbitrary number of fibers can be rapidly and continuously separated from the composite ribbon structure in a subsequent cabling or spooling operation without compromising the mechanical integrity of the sub-ribbons or the protective coating of the fiber. The sub-ribbons themselves are first formed in a multi-cavity die, passed through a curing oven, and then through an in-line die that applies the outermost matrix in a tandem operation. With this design/process, the fiber-km/hr throughput of the ribboning operation depends primarily on the maximum number of fibers that can be ribbonized in a single pass.
Thus, in accordance with the principles of the invention, the preferred embodiment may comprise a twenty-four fiber composite ribbon in which there are four sub-ribbons having six fibers each encased in a first matrix material, the characteristics of which are similar to, or the same as, those disclosed in the aforementioned DeFabritis et al patent. The sub-ribbons are arrayed side by side and the entire assembly is encased in a second matrix material which has a small thickness and low modulus and low tear strength, with sufficient adhesion to the first matrix material that their separation occurs by tearing through the web of the second material between or at the junction of, the sub-ribbons, with no effect on the robust sub-ribbons themselves. The thickness of the second matrix material, e.g., 10-30 xcexcm is less than the thickness of the first matrix material, being thin enough to be tearable. Both the forming and separating can be performed in a single pass operation.
These and other principles and features of the present invention will be more readily apparent from the following detailed description, read in conjunction with the accompanying drawings.